I don't know whether (a) I've got the wires connected wrong; or (b) I've got the wrong relay for the job; or (c) the relay is defective.

I have a constant 24v power supply on 30. I have my load (alternator exciter wire, a 24v alternator) on 87. I have a 12v switched power supply (from my ignition switch) on 85. 86 is connected to ground.

When I switch on the ignition, 30 and 87 are supposed to be connected so that the alternator gets excited.

I notice that there's a diode between 85 and 86 with some kind of connection to the the 30-87 circuit -- maybe that's putting 24v power where it shouldn't be? Maybe I need a relay without the diode and interconnection?

I'm not familiar with this relay and 24v alternators in general, but... Normally the ignition circuit (12v) is connected to the AUX terminal (and ground) on the alternator (I'm only familiar with 12V alternators), through an idiot light on the instrument panel. When 12v power is applied through the ignition switch it closes the relay switch to provide excitation current to the alternator, and the idiot light is illuminated. As soon as the alternator output has started, the AUX terminal raises to normal output voltage and no current flows through the ignition circuit, turning off the idiot light and opening the relay circuit shutting off the excitation current.

The diode will be fried now due to being connected backwards as previously mentioned. The relay will need to be replaced.

The purpose of the diode is to prevent a high voltage appearing across the coil of the relay when the relay is switched off. When the current through the coil is suddenly interrupted by opening the switch in the coil circuit the collapsing magnetic field of the coil (an inductor) wants to cause current to flow in the opposite direction to normal. Without the diode to allow that current to flow there is no place for the current to go due to the switch being open so the voltage across the coil rises until an arc occurs at the switch contact (normally) and the current is discharged.
With the diode present the current is able to flow through it and 'free-wheels' around the circuit formed by the diode and coil until the energy is dissipated in the coil. Because the current has somewhere to go the voltage across the coil only rises very minimally and arcing is prevented at the switch.

I'm not familiar with this relay and 24v alternators in general, but... Normally the ignition circuit (12v) is connected to the AUX terminal (and ground) on the alternator (I'm only familiar with 12V alternators), through an idiot light on the instrument panel. When 12v power is applied through the ignition switch it closes the relay switch to provide excitation current to the alternator, and the idiot light is illuminated. As soon as the alternator output has started, the AUX terminal raises to normal output voltage and no current flows through the ignition circuit, turning off the idiot light and opening the relay circuit shutting off the excitation current.

John

That's exactly how the regular 12v alternator, the one which came with the engine, works. That alternator powers the engine start battery, which is 12v.

The relay in question here is for exciting the second, large, school-bus alternator which provides power to my house batteries, which are 24v.

The diode will be fried now due to being connected backwards as previously mentioned. The relay will need to be replaced.

The purpose of the diode is to prevent a high voltage appearing across the coil of the relay when the relay is switched off. When the current through the coil is suddenly interrupted by opening the switch in the coil circuit the collapsing magnetic field of the coil (an inductor) wants to cause current to flow in the opposite direction to normal. Without the diode to allow that current to flow there is no place for the current to go due to the switch being open so the voltage across the coil rises until an arc occurs at the switch contact (normally) and the current is discharged.
With the diode present the current is able to flow through it and 'free-wheels' around the circuit formed by the diode and coil until the energy is dissipated in the coil. Because the current has somewhere to go the voltage across the coil only rises very minimally and arcing is prevented at the switch.

The diode is known as a free-wheeling diode in this application.

Excellent answer!

I was about to say the same thing , but decided I would read the other responses first.

Relays come in three varieties basic no resistor or diode, with a resistor, or with a diode. Th basic and the suppression resistor type are not polarity sensitive, and the diode type is.

In a diode protected relay they are polarity sensitive, (+) power goes to the cathode side pin, and ground goes to the Anode side pin. Look for a relay diagram or a relay schematic that is printed on the side of the relay to determine.

ON most Bosh Relays in the States 85 is the cathode side pin, ie positive. But this is not the case with all relays some use 86 as the cathode side.

I just got a batch of Cole Hersey Diode/Relays the diagram clearly showed 85 as the cathode. When I installed them, pop goes the fuse in the circuit, as well as the diode in the relay. I was pissed, So I used a little trick to test the rest of the relays in the batch before I installed them.

Trick='s nine volt battery to jump the relay, the cathode side will be evident as it will only turn on when the + is connected to the cathode side. And will not work when connected opposite polarity, neither will the 9 volts, pop the diode when connected reversed polarity. I tested 18 relays in this batch and all were ms-marked as to the cathode/Anode.

Also, if you are using the 30, and 87/87A to power and inductive coil, you better put in a free wheeling diode in that circuit as well.

I sure know a lot more about relays, than I did before. Thanks to all of you for the education

You've actually learnt a bit about inductive loads and about one way to prevent arcing, all of which is routine stuff if you're in the electricalgame.

The point made about also installing a diode across the relay contact is to prevent arcing at the contact caused by an inductive load (such as your alternator exciter coil) when the relay contact opens. This will extend the life of the relay.

You've actually learnt a bit about inductive loads and about one way to prevent arcing, all of which is routine stuff if you're in the electrical game.

The point made about also installing a diode across the relay contact is to prevent arcing at the contact caused by an inductive load (such as your alternator exciter coil) when the relay contact opens. This will extend the life of the relay.

Yep, not only saving the relays, but also any other ECU's and discrete built electronics from voltage spikes in both the 12 and 24 volt wiring.

I've seen expensive units taken out from the voltage spikes, many don't have on-board suppression.

The reverse connection may or may not have fried the diode but the relay should still work. When the diode is fried by over-current in the forward direction it acts like a fuse and OPENS, it is extremely unlikely it would short.

In the past we have deliberately removed the internal diode in relays by blowing them out with forward current. It worked 100% of the time.

When connected the "correct" way round do you still hear it "click". I suspect you have something else wrong with the wiring and replacing the relay will not cure it.